Research and development into improvements to automobile engines and the use of catalysts for cleaning exhaust fumes since the air pollution resulting from exhaust fumes became a serious social problem, and the prevention of air pollution is regarded as important.
As a result of the research and development, notable achievements have been made in the cleaning of exhaust fumes with a catalyst which have been put into practice to clean exhaust fumes from automobiles for removing pollution with a catalyst. Generally, the source from which nitrogen oxides are generated by the combustion of petroleum, coal, etc., are classified roughly into two main groups. One group is of the fixed source type, such as boilers, while the other group is of the moving source type, such as automobiles. Catalysts for cleaning exhaust fumes from automobiles differ from industrial catalysts for cleaning waste gases from fixed sources in that they encounter severe reaction conditions, so that catalysts for the cleaning of exhaust fumes must satisfy the following conditions:
1. The catalysts must have high catalytic activity and heat resistance over a wide temperature range; PA1 2. The catalyst must have high catalyst poison resistance against S, Pb. or the like and be long-lived; PA1 3. The catalysts must have physical strengths high enough to withstand shocks in running; PA1 4. The catalysts must consist of materials which exist abundantly as natural resources and do not cause secondary pollution.
As to lead laden gasoline, serious difficult problems are not yet solved. On the other hand, lead-free gasoline has received practical acceptance, and now it is used primarily.
The practical application of lead-free gasoline has made possible the cleaning of exhaust fumes with a catalyst. Research and development into the cleaning of exhaust fumes with a catalyst has been active, and as a result of the research and development, the cleaning of exhaust fumes with a catalyst had produced practical applications.
As a catalyst for the cleaning of exhaust fumes, noble metal catalysts such as catalysts made from platinum (Pt), palladium (Pd), ruthenium (Ru) or the like, have already become practicable, and at present are used with remarkable success, although the use of noble metals is to be disfavored for various reasons. Needless to say, the noble metal catalyst ingredient is set on a ceramic carrier, such as (gamma).gamma.-alumina (Al.sub.2 O.sub.3), and titanium oxide (TiO.sub.2). Noble metal catalysts are high-priced and the amount of noble metals which lie under the ground is limited. Therefore, at present, the target of research and development in noble metal catalysts is toward heightening of catalytic activity per unit weight of noble metal catalysts, and suppressing the formation of ammonia (NH.sub.3) in a reducing atmosphere, and improving noble metal catalysts so that noble metal catalysts can have sufficiently high catalytic activity in the reducing atmosphere. Research and development is being made into improved recovery and reclamation of used noble metals and other matters also.
On the other hand, as a catalyst for removing nitrogen oxides from fixed sources, catalysts are also made from base metals such as iron (Fe), copper (Cu), and base metal oxides which exist abundantly as natural resources. These base metal catalysts are not used for cleaning of exhaust fumes. The reason for this is that base metal catalysts are inferior in their ability compared to that of noble metal catalysts, and because of the fear that the base metal catalyst ingredients, scattered in the air, could cause secondary pollution.
Nevertheless, research and development is actively being made into the use of base metals and base metal oxides as catalyst ingredients of a catalyst for cleaning exhaust fumes. Particularly noticeable materials among base metals and base metal oxides are Fe and Fe.sub.2 O.sub.3, which have very high initial catalytic activity for the NO reduction reaction. Therefore it is supposed that the problem will be solved if Fe and Fe.sub.2 O.sub.3 catalysts having high heat resistance and durability are developed. However, though much research and development has been made into base metal catalysts, the Fe and Fe.sub.2 O.sub.3 catalysts which satisfy the above-mentioned conditions have not been developed and they are far from being put into practical use.
Catalyst carriers are classified roughly into pellet-type carriers and honeycomb-type carriers. As manufacturing processes for a catalyst, the following processes are given: a process in which a catalyst ingredient and a catalyst carrier powder, usually with the particle diameter in the range of 1-10 .mu.m (micrometer), are mixed thoroughly and thereafter the obtained compound is formed; and a process in which a catalyst ingredient is deposited on a catalyst carrier by electroplating. As manufacturing processes for a catalytic ingredient, the following processes are given: a process in which the catalytic ingredient is ground to powder with a ball mill or the like; and a process in which a catalyst ingredient is formed by co-precipitation from a solution of a mixture containing the catalyst ingredient.